Genetic manipulation of gene expression in mammals holds great potential for functional studies of particular genes and their products, and in applications for drug discovery and gene therapy. An ideal gene regulation system would be low in basal activity, but highly and specifically responsive to the induction. In addition, the expression of a given gene should be dose-responsive, and the system could be reversibly switched on or off promptly. This is particularly valuable for gene therapy in which pharmacological control over timing and levels of a particular gene expression within a therapeutic range is critical for certain diseases.
Recently, several inducible systems for mammalian cells have been developed. These with their variants include FK506/rapamycin, RU488/mifepristone, ecdysone-inducible, and tetracycline (Tet) inducible systems (1-4). Currently, the Tet-inducible system is most commonly used for regulated gene expression in vivo. Significant improvements have been made in this Tet system to reduce its basal expression level and to improve its inducibility in vivo (5). However, one major shortcoming for this system is the lack of an effective antagonist for its inducers; the potent inducer doxycycline, for example, has a considerable half-life (about 24 hr) in vivo (6). This pharmacokinetic property may exclude its use in situations where prompt and efficient on/off switching is essential, such as for gene therapy or for precise regulated expression of specific genes during development (1).